ch16 - phototropism and geotropism

what is tropism

the growth of a plant in response to a stimuli

why is tropism referred to as directional growth

the orientation/ direction a plant grows in e.g. towards or away from the stimuli depends on direction/ angle of the stimulus

why is tropism beneficial for plants

enables plants to grow towards favourable stimuli which maximises growth and increases survival of the plant

what plant growth factor is involved in tropism and why

IAA - type of auxin

» made in shoots and roots by meristematic cells

» high conc stimulates cell elongation in shoots and inhibit cell elongation in roots

what is phototropism

growth in response to light (towards or away from light)

what are the 2 types of phototropism and what does each one cause

positive (+ve) phototropism - shoots bends and grows towards light

negative (-ve) phototropism - roots growing away from light

explain positive phototropism

IAA is produced in apical meristem and diffuses down the shoot tip > unilateral light stimulates IAA to diffuse to shaded side of shoot

» higher conc of IAA stimulates more cell elongation on shaded side therefore shaded side will grow faster = bends towards the light

explain negative phototropism

IAA is produced in root tip and diffuses along > unilateral light causes IAA to diffuse to shaded side of root however it inhibits cell elongation

» lower conc of IAA on unshaded side enables it to grow normally so more cell elongation in shaded side therefore grows faster = bends down away from light

what is geotropism

growth in response to gravity (towards or away from gravity)

what is another name for geotropism

gravitropism

what are the 2 types of geotropism and what does each one cause

positive (+ve) geotropism - roots growing downwards towards gravitational pull

negative (-ve) geotropism - shoots growing upwards away from gravitational pull

explain positive gravitropism

IAA diffuses to lower side of shoot > high conc of IAA in root inhibits cell elongation on lower side

» upper side has lower conc of IAA so cell elongation is not inhibited therefore cell elongation occurs normally which causes faster growth in upper side therefore grows downwards towards gravity

explain negative gravitropism

IAA diffuses to lower side of shoot > high conc of IAA in shoot stimulates cell elongation on the lower side

» lower side grows faster than upper side causing shoot to grow against gravity upwards

why is positive phototropism beneficial

enables shoots and leaves to grow towards light to absorb light for photosynthesis

why is positive gravitropism beneficial

enables roots to grow downwards to anchor the plant into the soil + to absorb water and mineral ions

what is etiolation

when a seed grows really rapidly in darkness

what occurs during etiolation

since a seed cant perform photosynthesis, embryo uses glucose during cellular respiration to generate ATP resulting in extremely rapid growth > gibberellins and auxins work synergistically for fast cell and stem elongation

why does an etiolated plant: look white/ yellow, small unexpanded leaves and spindly

building leaves and synthesizing chlorophyll requires lots of energy - by not carrying out these processes energy is prioritised for vertical growth

grows thin and spindly to try and grow whereever light may be

what happens once plant reaches light

leaves expand

green chloroplast develops

elongation rates decrease